In a cable television system (CATV), television programs are provided at a central head-end. The programs are distributed from the head-end through branching tree-like networks of optical fibers to a multitude of hybrid fiber cable nodes (HFCNs) in respective local communities. Then further distributed from the HFCNs through branching tree-like networks of coaxial cables to customer interface units (CIUs), also called cable terminations.
Currently, many of these systems are beginning to provide additional communication services such as telephone services and computer networking services (e.g. internet connection) through the cable television system. Telephone and computer networking services require bi-directional communication in the cable television system. Forward data signals for these additional services are transmitted in a manner similar to television signals, as described above, and return data signals are transmitted through the same path in the reverse direction. That is, return signals are collected from the CIUs through the branching coaxial cable networks to the HFCNs, back through the HFCNs, and back through the branching optical fiber network to the head-end.
At the head-end, a multitude of electronic forward information signals for broadcast television and additional services (telephone and computer communications) are used to modulate respective carrier signals of different frequencies. The modulated carrier signals are combined into an electronic multi-carrier forward signal that is used to modulate a forward laser beam to produce an optical forward signal carried by the forward laser beam. The modulated laser beam, carrying the optical forward signal, is transmitted through the optical fiber network to a multitude of the HFCNs. At each local node an optical detector coverts the optical forward signal back into an electronic forward signal. The reconverted electronic forward signal is transmitted from the HFCNs through the coaxial cable network to CIUs at homes and businesses of customers.
At the cable termination, telephone and computer equipment of the customer, are connected to the CIUs. The customer's equipment produce electronic return signals that are transmitted by the CIUs into the coaxial cable network. The return signals are multi-carrier modulated signals similar to the forward signals. The return signals travel back through the tree-like coaxial cable network to the HFCNs. In the HFCNs, the return signals are separated from the forward signals by diplex filters. The separated return signals are used to modulate a return laser beam to produce a multi-carrier optical return signal carried by the return laser beam. The optical return signal is transmitted back through the tree-like optical fiber network to the head-end where the optical return signals are converted back into electronic return signals by an optical detector for the return signals. The electronic return signals are demodulated and used for telephone and computer communications.
Requirements for signal to noise ratio (S/N) at the cable termination together with limits on the allowed optical power, limit the length of one-directional optical transmission of analog television signal to around 100 km. In the coaxial cable network, line amplifiers are required at intervals of approximately 300 to 350 meters in order to maintain the amplitude of the high frequency electronic signals. The line amplifiers in the coaxial cable network produce distortions that result in additional noise that further limits the length of signal transmission.
In bi-directional transmission, the introduction of return light beams in the optical fiber network results in crosstalk as additional noise that further reduces the range of cable broadcasting. The line amplifiers must be bi-directional and both the forward and return amplifiers produce distortions that result in increased noise in both the forward and return directions which further limits transmission distance.
An important part of the distortion caused by power amplifiers is the composite triple beat (third order) distortion. In addition to the two amplifiers in each bi-directional line amplifier, the optical transmitters, optical receivers, and CIUs each include a power amplifier. The distortions are cumulative as the signal passes through a multitude of power amplifiers from the source of the signal to the CIUs, and the distortions from return signal amplification in the line amplifiers also adds to the distortion of the forward signals. The result is that signal transmission in bi-directional systems is even more limited by noise than in previous one-directional systems.
Those skilled in the art are directed to the following citations. U.S. Pat. No. 4,947,386 to Preschutti discloses a broadband network with a bi-directional amplifier. U.S. Pat. No. 5,343,158 to Gris discloses another bi-directional amplifier. U.S. Pat. No. 5,519,434 in FIG. 2 discloses an all pass filter.
The above references are hereby incorporated herein in whole by reference.